bug-4137 enable possibility to predefine variables

This is proposition for a general solution for solving bug #4137

For example on CentOS 7 to create XFS filesystem on block device set in the environment or befo the script is called:

MKFS="sudo /sbin/mkfs"

(cherry picked from commit 584aca535bf9835c0b4c9aa2c892f36a7b312fcf)

src/mad/sh/scripts_common.sh

@@ -18,43 +18,43 @@ export LANG=C
 
 # Paths for utilities
 export PATH=/bin:/sbin:/usr/bin:$PATH
-AWK=awk
-BASH=bash
-CUT=cut
-CEPH=ceph
-DATE=date
-DD=dd
-DF=df
-DU=du
-GREP=grep
-ISCSIADM=iscsiadm
-LVCREATE=lvcreate
-LVREMOVE=lvremove
-LVRENAME=lvrename
-LVS=lvs
-LN=ln
-MD5SUM=md5sum
-MKFS=mkfs
-MKISOFS=genisoimage
-MKSWAP=mkswap
-QEMU_IMG=qemu-img
-RADOS=rados
-RBD=rbd
-READLINK=readlink
-RM=rm
-SCP=scp
-SED=sed
-SSH=ssh
-SUDO=sudo
-SYNC=sync
-TAR=tar
-TGTADM=tgtadm
-TGTADMIN=tgt-admin
-TGTSETUPLUN=tgt-setup-lun-one
-TR=tr
-VGDISPLAY=vgdisplay
-VMKFSTOOLS=vmkfstools
-WGET=wget
+AWK=${AWK:-awk}
+BASH=${BASH:-bash}
+CUT=${CUT:-cut}
+CEPH=${CEPH:-ceph}
+DATE=${DATE:-date}
+DD=${DD:-dd}
+DF=${DF:-df}
+DU=${DU:-du}
+GREP=${GREP:-grep}
+ISCSIADM=${ISCSIADM:-iscsiadm}
+LVCREATE=${LVCREATE:-lvcreate}
+LVREMOVE=${LVREMOVE:-lvremove}
+LVRENAME=${LVRENAME:-lvrename}
+LVS=${LVS:-lvs}
+LN=${LN:-ln}
+MD5SUM=${MD5SUM:-md5sum}
+MKFS=${MKFS:-mkfs}
+MKISOFS=${MKISOFS:-genisoimage}
+MKSWAP=${MKSWAP:-mkswap}
+QEMU_IMG=${QMEMU_IMG:-qemu-img}
+RADOS=${RADOS:-rados}
+RBD=${RBD:-rbd}
+READLINK=${READLINK:-readlink}
+RM=${RM:-rm}
+SCP=${SCP:-scp}
+SED=${SED:-sed}
+SSH=${SSH:-ssh}
+SUDO=${SUDO:-sudo}
+SYNC=${SYNC:-sync}
+TAR=${TAR:-tar}
+TGTADM=${TGTADM:-tgtadm}
+TGTADMIN=${TGTADMIN:-tgt-admin}
+TGTSETUPLUN=${TGTSETUPLUN:-tgt-setup-lun-one}
+TR=${TR:-tr}
+VGDISPLAY=${VGDISPLAY:-vgdisplay}
+VMKFSTOOLS=${VMKFSTOOLS:-vmkfstools}
+WGET=${WGET:-wget}
 
 if [ "x$(uname -s)" = "xLinux" ]; then
     SED="$SED -r"